my Macbook air stop connecting to the internet one day after it was doing so with no problem.

I understand your Mac spontaneously failed to connect to the Internet for reasons you can't explain.

The basic instructions are as follows but it probably isn't telling you anything you don't already know: Connect your Mac to the internet - Apple Support.

Assuming you're doing everything it says, try creating a new Network Location according to the following: How to use network locations on your Mac - Apple Support.

If the problem is resolved using that Network Location, delete the problematic one.

If not, Wireless Diagnostics may reveal some suggestions: Use Wireless Diagnostics on your Mac - Apple Support. Its Summary page at the end may be its most useful result. Normally, only Review Wi-Fi Best Practices is its sole entry. Anything in addition to that requires investigation. Please write back and describe its results, or post a screenshot.